The present invention relates to an electronic tuner for use in high frequency circuit devices, such as a cable television receiver, a television receiver and a video tape recorder.
For example, a tuner circuit of a television receiver or the like selects a signal of a desired channel from television signals in the VHF or UHF band received using an antenna, converts it into an intermediate frequency and supplies the resultant signal to a demodulation circuit located in the succeeding stage. Here, the CATV system can transmit tens to a hundred and tens of programs, and a CATV receiver for receiving these programs uses a double conversion-type electronic tuner that is particularly effective for receiving multi-channel programs.
For instance, as shown in FIG. 18, in the above-mentioned conventional tuner 101, a RF (Radio Frequency) signal received by the antenna is input to an up-converter 103 via a band-pass filter 102a, a PIN attenuator 102b and a RF amplifier 102c, and mixed with a first local oscillator signal Lo1 of a frequency selected according to a received channel by a first mixer circuit 103 of the up-converter 103. Thus, the RF signal is converted into a first intermediate frequency signal IF1 of a higher frequency.
Moreover, the first intermediate frequency signal IF1 is input to a first IF signal amplifying circuit 105 via a band-pass filter 104. After the first intermediate frequency signal IF1 is amplified by the first IF signal amplifying circuit 105, it is applied to a down-converter 107 via a band-pass filter 106. In the down-converter 107, the output of the band-pass filter 106 is mixed with a second local oscillator signal Lo2 in a second mixer circuit 171 as in the up-converter 103. Hence, the first intermediate frequency signal IF1 is converted into a second intermediate frequency signal IF2 of a frequency lower than the RF signal, and output via a band-pass filter 108 and a second IF signal amplifying circuit 109.
Here, in order to make the level of the second intermediate frequency signal IF2 output by the electronic tuner 101 constant irrespective of variations in the strength of the received signal, an automatic gain control (AGC) signal is applied to the PIN attenuator 102b to control the amount of attenuation. Incidentally, the first (second) local oscillator signal is generated by a first (second) local oscillator circuit 133 (173), amplified by a local oscillator signal amplifying circuit 132 (172), and then applied to the first (second) mixer circuit 131 (171).
The double conversion-type electronic tuner 101 with the above-described structure increases the frequency of the RF signal by the up-converter 103 and decreases the frequency by the down-converter 107. It is therefore possible to select a desired channel even from a CATV broadcast that transmits multi-channel programs while efficiently removing disturbance.
Here, in the electronic tuner 101, for example, the circuits following the up-converter 103 are configured as shown in FIG. 19. Specifically, the first IF signal amplifying circuit 105 includes a bipolar transistor T151 with a grounded emitter, and the output of the first IF signal amplifying circuit 105 is input to the second mixer circuit 171 of the down-converter 107 via the band-pass filter 106 composed of a double-tuned circuit. Moreover, the second mixer circuit 171 includes a bipolar transistor T171 with a grounded base, and the second local oscillator signal Lo2 is input together with the first intermediate frequency signal IF1 to the emitter of the bipolar transistor T171 serving as an input terminal and converted into the second intermediate frequency signal IF2. The second intermediate frequency signal IF2 is input to a second IF signal amplifying circuit 109 via a band-pass filter 108 composed of a double-tuned circuit like the band-pass filter 106.
However, in the above-described conventional structure, in order to ensure disturbance characteristics durable to the input of a greater number of channels because of the digitalization of the CATV broadcast, the power consumed by the electronic tuner is increased and the inner temperature of the electronic tuner rises considerably. Moreover, it is necessary to add a digital signal processing circuit to meet the digitalization of the CATV broadcast, and consequently the heating value of the electronic tuner is increased in comparison with an electronic tuner configuration for receiving only an analog broadcast. As a result, the range of temperature variations in the electronic tuner is increased in comparison with the electronic tuner configuration for receiving only an analog broadcast, causing a problem that the electrical performance such as the gain and distortion characteristics of the electronic tuner deteriorates.
For instance, in the electronic tuner 101 with the configuration shown in FIG. 19, the gain in the circuits 151 and 171 including the bipolar transistor T151 (171) is varied with a change in temperature. Therefore, for example, as shown in FIG. 20, the level of the second intermediate frequency signal IF2 output by the electronic tuner 101 is varied within a range of around 24 [dB] to around 35 [dB] in a frequency band from 100 [MHz] to 800 [MHz] at 10xc2x0 C., 25xc2x0 C. and 60xc2x0 C.
An object of the present invention is to provide an electronic tuner of high quality by reducing deterioration of the electrical characteristics of the electronic tuner caused by a rise in temperature, without adding a complicated temperature compensating circuit.
In order to achieve the above object, an electronic tuner of the present invention includes:
an up-converter for converting an input high frequency signal into a first intermediate frequency signal of a frequency higher than the high frequency signal;
a down-converter for converting the first intermediate frequency signal into a second intermediate frequency signal of a frequency lower than the high frequency signal;
a high frequency amplifying circuit, disposed on a signal transmission path between an input of the high frequency signal and an output of the second intermediate frequency signal as an output signal, for amplifying an input signal; and
a temperature compensating section for compensating for deterioration of electrical characteristics caused by a change in temperature.
According to this structure, even when the ambient temperature is changed a lot as in a digital CATV broadcast receiver in comparison with an analog broadcast receiver, it is possible to achieve an electronic tuner that has less deterioration of electrical performance such as the gain and distortion characteristics of the electronic tuner and has a simple circuit structure by providing the temperature compensating section.
It is preferable that the electronic tuner further includes a filter circuit, disposed on the signal transmission path between an input of the high frequency signal and an output of the second intermediate frequency signal as an output signal, for passing frequency components in a predetermined band,
the high frequency circuit includes a field-effect transistor as an amplifying element, and
the temperature compensating section includes:
a thermistor that is disposed in a preceding stage or succeeding stage of the filter circuit and has such a characteristic that resistance is decreased in substantially proportion to a rise in temperature within a predetermined temperature range; and
a serial circuit composed of the thermistor and a coupling capacitor of the filter circuit.
According to this structure, the high frequency amplifying circuit includes the field-effect transistor as an amplifying element, and the temperature compensating section includes the thermistor, which is disposed in the preceding stage or succeeding stage of the filter circuit and has such a characteristic that resistance is decreased in substantially proportion to a rise in temperature within a predetermined temperature range, and the serial circuit composed of the thermistor and the coupling capacitor of the filter circuit. Incidentally, the high frequency amplifying circuit may be a mixer circuit, or a high frequency circuit for amplifying the high frequency signal, or first or second intermediate frequency signal.
In this structure, the gain in the high frequency amplifying circuit having the field-effect transistor is decreased in substantially proportion to the temperature. On the other hand, since the resistance of the thermistor is decreased in substantially proportion to a rise in temperature, the degree of coupling between the circuit provided in the preceding stage of the temperature compensating section and the circuit provided in the succeeding stage is increased. As a result, the level of the signal output by the temperature compensating section is increased, and thereby compensating for a decrease of the gain in the high frequency amplifying circuit. Here, the coupling capacitor of the filter circuit is also used for temperature compensation. Consequently, it is possible to achieve an electronic tuner that provides a substantially constant output signal level within the above-mentioned temperature range by a simple circuit structure obtained by only adding the thermistor. As a result, for example, even when the present invention is applied to an electronic tuner in which the temperature tends to increase as in a multi-channel digital CATV broadcast receiver in comparison with an analog broadcast receiver or a receiver for a small number of channels, it is possible to realize a high-quality electronic tuner that limits the deterioration of electrical characteristics such as the gain and distortion characteristics.
Furthermore, it is preferable that the filter circuit is a double-tuned circuit including a primary resonant circuit and a secondary resonant circuit, between which the resonant circuit on the thermistor side has a resonant capacitor with a temperature characteristic that is set to achieve impedance matching with the thermistor, coupling capacitor and high frequency amplifying circuit even when temperature is changed.
According to this structure, since the temperature characteristic of the resonant capacitor is set as described above, it is possible to limit the occurrence of matching loss resulting from a change in temperature. As a result, despite a simple circuit structure obtained by only adding the thermistor, it is possible to provide a high-quality electronic tuner that achieves a high S/N ratio.
Besides, in the structure where the high frequency amplifying circuit includes a dual-gate field-effect transistor as an amplifying element, it is preferable to use a thermistor that is disposed between a gain control terminal of the dual-gate field-effect transistor and a negative power supply line and has such a characteristic that the resistance is increased in substantially proportion to a rise in temperature within a predetermined temperature range as a thermistor in the temperature compensating section.
According to this structure, even when the temperature compensating section of the present invention is provided, for example, in the mixer circuit of the up-converter, it is possible to provide the same effect.
Moreover, the electronic tuner of the present invention may include a plurality of temperature compensating sections.
For instance, in the above structure, the up-converter or down-converter may include:
a local oscillator circuit for generating a local oscillator signal of a predetermined frequency;
a local oscillator signal amplifying circuit for amplifying an output of the local oscillator circuit;
a mixer circuit for mixing an output of the local oscillator signal amplifying circuit and an input signal so as to generate the first intermediate frequency signal; and
a second temperature compensating section, provided on at least a signal transmission path between the local oscillation circuit and the mixer circuit, for compensating for deterioration of electrical characteristics caused by a change in temperature.
With this structure, it is possible to further limit the deterioration of the electronic tuner caused by a change in temperature in comparison with a structure including only one temperature compensating section.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.